1. Field of the Invention
The present invention relates to a two-stroke engine.
2. Description of the Related Art
In a known two-stroke engine, three exhaust valves are arranged on the peripheral portion of the inner wall of the cylinder head, and a pair of intake valves are arranged on the peripheral portion of the inner wall of the cylinder head at a position opposite to the exhaust valves. The valve opening of each intake valve, which is located on the exhaust valve side, is masked by the masking wall to prevent fresh air from flowing out from the valve opening of each intake valve, which is located on the exhaust valve side. The fresh air flowing out from the unmasked valve opening of each intake valve, which is located on the opposite side of the exhaust valve, is caused to flow downward along the inner wall of the cylinder bore beneath the intake valve. Then, the fresh air flows along the top face of the piston and then flows upward along the inner wall of the cylinder bore beneath the exhaust valve (see Japanese Unexamined Patent Publication No. 4-112904). This two-stroke engine is designed so as to scavenge burned gas as efficiently as possible by causing the fresh air flowing out from the intake valves to flow along the periphery of the combustion chamber in the form of a loop.
However, if the fresh air is caused to flow along the periphery of the combustion chamber in a looplike manner, the unburned gas existing in the periphery of the combustion chamber can be sufficiently scavenged by the fresh air, but the unburned gas existing at the central portion of the combustion chamber is not scavenged by the fresh air and thus stays in the combustion chamber. As a result, a problem arises in that it is impossible to sufficiently scavenge all of the unburned gas in the combustion chamber.